1. Field of the Invention
This invention relates to a fluid bearing adapted to support a rotary body such as a rotating shaft of a machine tool in a non-contact fashion by means of the static or dynamic pressure of the fluid found in the bearing clearances of the bearing and also to a rotary drive apparatus using such a fluid bearing.
2. Related Background Art
Fluid bearings including gas bearings such as static pressure gas bearings and dynamic pressure gas bearings are advantageous in that the movable member such as a rotating shaft supported by the bearing can move lightly, smoothly and accurately because the bearing produces little frictional resistance. Therefore, such bearings are popularly used with high speed rotating shafts of precision machine tools and other tools that require an enhanced level of operational precision.
Generally, static pressure gas bearings have orifices for releasing a jet stream of gas into the bearing clearances between the bearing and the rotary body supported by it, which orifices may be of the self-formation type, the surface type or the porosity type. The bearing of this type structurally comprises a radial bearing section for radially supporting the rotary body and thrust bearing sections for axially supporting the rotary body, which bearing sections define respective bearing clearances with the rotor and the thrust plates of the rotary body that face them respectively, into which a jet stream of gas is released through any of the orifices in order to support the rotary body in a floating state. The bearing clearances of a static pressure gas bearing are very narrow and normally between several microns and tens of several microns.
In the case of dynamic pressure gas bearings, on the other hand, helical or herring-bone-shaped dynamic pressure generating grooves that are several to tens of several microns deep are formed on the rotor and the thrust plate of the rotary body supported by the bearing or the bearing surfaces facing them and dynamic pressure is generated by drawing the air in the bearing clearances between the rotor and the thrust plate of the rotary body and the corresponding bearing surfaces into the dynamic pressure generating grooves in order to support the rotary body in a floating state. As in the case of static pressure gas bearings, the bearing clearances of a dynamic pressure gas bearing are very narrow and normally between several microns and tens of several microns.
Note that the rotor and the thrust plate of a rotary body supported by a fluid bearing may be partly integral with each other or bonded to each other by means of one or more than one bolt.
FIG. 1 of the accompanying drawings is a schematic cross sectional view of a known fluid bearing having a configuration as described above. Referring to FIG. 1, the rotary body comprises a rotor 111 having a cylindrical surface to be borne on the bearing and a pair of thrust plates 112 bonded respectively to the opposite ends of the rotor 111 by means of bolts (not shown). The rotary body is rotatably supported by a bearing housing 101 in a non-contact fashion. The bearing housing 101 comprises a radial bearing section having a pair of porous radial bearing pads 102 having surfaces arranged vis-à-vis the cylindrical surface of the rotor 111 of the rotary body and a pair of thrust bearing sections having respective porous thrust bearing pads 103 having surfaces arranged vis-à-vis the respective surfaces of the thrust plates 112. The radial bearing pads 102 and the thrust bearing pads 103 are rigidly secured to the bearing housing 101 by means of shrinkage fitting, adhesive or some other appropriate measure.
Pressurized gas fed from a high pressure gas supply source (not shown) is delivered to each of the radial bearing pads 102 and the thrust bearing pads 103 by way of pressurized gas supply port 101a of the bearing housing 101 and made to hit the surfaces of the rotor 111 and the thrust plates 112 facing the respective bearing sections before being moved away to the outside by way of the outer periphery of the surfaces of the thrust plates facing the bearing sections and the exhaust port 101b. The rotary body is supported by the bearing in a non-contact fashion under the effect of the static pressure of the fluid injected into the bearing clearances.
The thrust plates 112 and the rotor 111 are provided with a central through hole formed around the axis of rotation of the rotary body and the inner peripheral surfaces 112a of the through holes of the thrust plates 112 define a uniform inner diameter A0.
However, the above described known fluid bearing has a drawback that, as the gas bearing is driven to rotate at high speed, the rotor and the thrust plates are deformed by the generated centrifugal force to allow the thrust plates to fall and locally reduce the sizes of the bearing clearances. As a result, problems such as scoring and seizure occur. More specifically, a rotating cylindrical or disk-shaped member produces an outward displacement that is proportional to the outer diameter thereof so that the thrust plates having an outer diameter greater than that of the rotor is displaced much more than the latter to consequently change the sizes of the bearing clearances.
FIG. 2 of the accompanying drawings schematically shows the rotor and the thrust plates in cross section when they are stopped (as indicated by S1) and when they are rotating (as indicated by S2). It is known that, as the rotor and the thrust plates that are dimensionally differentiated from the rotor are bonded to each other and driven to rotate, the thrust plates fall toward the rotor. In other words, since the thrust plates that are trying to move outward are arrested by the rotor at the bonded areas, the rotor that is pulled by the thrust plates expands at and near the bonded areas while the thrust plates that are pulled by the rotor tend to fall onto the rotor. In FIG. 2, R denotes the axis of rotation of the rotary body.
As the rotary body is deformed in this way, the bearing clearances of the radial bearing section and the thrust bearing sections are locally dimensionally reduced so that those sections eventually contact the rotor and the thrust plates to give rises to problems such as scoring and seizure. If the bearing clearances are dimensionally reduced too much, those sections easily come to contact the rotary body to make the gas bearing inoperative once the gas bearing is driven to rotate.
The problem of falling thrust plates may be avoided by arranging an object having a profile same as the rotor to the other side of each of the thrust plates. However, the other side is often used to arrange there a motor or a jig whose profile is mostly determined as a function of the operating conditions and the purpose of its use so that it is highly difficult to make it show a profile that is optimal for suppressing the deformation of the thrust plates.
Meanwhile, there have been proposals for designing the parts of the rotor and the thrust plates that are apt to be deformed most or the corresponding parts of the bearing so as to accommodate the deformation produced as a result of the falling phenomenon of the thrust plates. Japanese Patent Application Laid-Open No. 63-176817 describes such a rotor while Japanese Patent Application Laid-Open No. 2711584 discloses such a thrust plate.
However, the bearing clearances are designed to show an optimal profile after the rotor and the thrust plates are deformed as a result of rotation of the rotary body. In other words, they do not show an optimal profile when the rotary body is stationary and therefore the gas bearing performs only poorly at the time when the rotary body starts rotating or when it is rotating at low speed.
In view of the above circumstances, it is therefore the object of the present invention to provide a fluid bearing that can effectively prevent the phenomenon of falling thrust plates that can be caused by the centrifugal force generated when rotating at high speed without sacrificing the performance of the bearing at the start and during the low speed rotary operation and also a rotary drive apparatus using such a fluid bearing.
According to the invention, the above object is achieved by providing a fluid bearing comprising:
a rotary body including a rotor having a cylindrical surface to be borne and at least a thrust plate arranged at an end of the rotor in the sense of axis of rotation; and
a housing including a radial bearing section arranged vis-à-vis said surface of said rotor and a thrust bearing section arranged vis-à-vis said thrust plate;
said thrust plate having a hole bored around the axis of rotation of the rotary body, the inner diameter of said hole being greater at the rotor side than at the opposite side.
In another aspect of the invention, there is also provided a fluid bearing comprising:
a rotary body including a rotor having a cylindrical surface to be borne and at least a thrust plate arranged at an end of the rotor in the sense of axis of rotation; and
a housing including a radial bearing section arranged vis-à-vis said surface of said rotor and a thrust bearing section arranged vis-à-vis said thrust plate;
the outer diameter of said thrust plate being greater at the rotor side than at the opposite side.
In still another aspect of the invention, there is provided a rotary drive apparatus comprising a fluid bearing according to the invention and a motor for driving said rotary body to rotate.